The present invention relates to land-based or industrial gas turbines, for example, for electrical power generation, and particularly to a cooling circuit for a nozzle stage of the gas turbine.
Traditionally, discharge air is extracted from the compressor of a turbine for purposes of cooling turbine blades and nozzles. It has also been recognized that hot gas path components of the gas turbine can be cooled by flowing cooling steam in heat exchange relation with the surfaces to be cooled. Combined steam and air-cooling of nozzles in a gas turbine has been proposed, for example, in U.S. Pat. No. 5,634,766, of common assignee herewith. In that patent, steam is supplied to a plenum in the outer wall containing an impingement plate with openings for flowing steam through the impingement plate openings against the interior wall surface of the outer wall to cool the latter. The steam then flows into a pair of cavities in the vane and particularly through inserts in the cavities having apertures for impingement-cooling of the surrounding interior walls of the vane. The spent impingement steam flows into a plenum in the inner wall for flow through openings in another impingement plate to impingement-cool the inner wall. Return steam flows through cavities containing insert sleeves having openings for impingement-cooling the adjacent walls of the vane. Air-cooling is supplied to a trailing edge cavity for flow through openings in the trailing edge into the hot gas stream.
While that cooling system is satisfactory, experience has shown that thermal barrier coatings on the leading edges of the vanes tend to erode. Very high thermal gradients thus occur when the nozzle leading edge is cooled from the back side without external insulation along the leading edge. Resulting thermal stresses produce a shortfall in low-cycle fatigue lives. Also, because of the high thermal gradients at the leading edge eroded areas, the nozzle requires a leading edge metal thickness with tight tolerances on wall thickness variations. This significantly increases manufacturing costs and produces high scrap rates. Further, the inner and outer walls of the cooling system of U.S. Pat. No. 5,634,766 require covers serving, in part, as manifolds for the steam supplied to the nozzles. The covers are welded to the bands and the weld joint experiences high thermal stress due to the difference in temperature between the cover running at steam temperature in comparison with the temperature of the nozzle bands. There has thus developed a need for a turbine nozzle cooling system which alleviates the above and other problems associated with cooling turbine nozzles.
In accordance with a preferred embodiment of the present invention, combined steam and air-cooling of nozzles are provided, with air-cooling in part being provided by film-cooling in the hot gas path. To accomplish this, each nozzle vane is comprised of a plurality of cavities extending the length of the vane between the leading and trailing edges of the vane. Compressor discharge air is directed through an impingement plate for impingement against the outer wall surface of the outer wall to cool the outer wall. Post-impingement air then flows through cooling holes formed in the outer wall about the vane, producing a layer or film of cooling air on the radially inner wall surface of the outer wall, i.e., forming a film along the outer band wall in the hot gas flow path. Cooling air is also directed through an insert sleeve extending lengthwise in a leading edge cavity of the vane. The insert sleeve has openings for impingement-cooling of the leading edge. Post-impingement cooling air flows outwardly through holes in the leading edge to form a film flow about the leading edge of the vane in the hot gas path. Air also flows in a trailing edge cavity for flow through openings in the side walls of the trailing edge to form a cooling film flow along the side walls of the trailing edge. Air in the cavity also passes through holes in the trailing edge tip for flow outwardly directly into the hot gas path.
Cavities intermediate the leading and trailing edge cavities are provided with steam for cooling the side surfaces of the vane between the inner and outer walls. Particularly, a steam inlet supplies steam through insert sleeves having openings for impingement-cooling the side walls of the vane. The post-impingement steam flows into a plenum in the inner band for flow through an impingement plate to cool the inner wall. The cooling steam then flows outwardly through insert sleeves in the remaining intermediate cavities of the vane for flow through openings for impingement-cooling of the side surfaces of the vane. An outlet for these remaining cavities carries the spent cooling steam. Consequently, thin-film cooling is provided in combination with steam-cooling of the nozzles.
In a preferred embodiment according to the present invention, there is provided a turbine vane segment comprising inner and outer walls spaced from one another, a vane extending between the inner and outer walls and having leading and trailing edges, the vane including a plurality of discrete cavities between the leading and trailing edges and extending lengthwise of the vane for flowing cooling mediums, an impingement plate having openings therethrough and spaced outwardly of the outer wall defining a chamber with the outer wall for receiving cooling air through the impingement plate openings for impingement-cooling the outer wall, the outer wall having a plurality of holes for flowing post-impingement air from the chamber holes therethrough for film-cooling the outer wall along a hot gas path of the turbine and a pair of the cavities comprising cavities adjacent the leading edge and the trailing edge, respectively, for flowing cooling air to cool respective leading and trailing edges, at least two of the plurality of cavities disposed between the leading and trailing edge cavities and having insert sleeves therein, the sleeves extending substantially between the inner and outer walls and having openings therethrough, the inner wall including a plenum and the two cavities lying in communication with one another through the plenum, the outer wall having an inlet for flowing steam into one of the two cavities and an outlet for flowing spent cooling steam from another of the two cavities, the steam in the two cavities flowing through the openings in the insert sleeves for impingement-cooling side walls of the vane.
In a further preferred embodiment according to the present invention, there is provided a turbine vane segment comprising inner and outer walls spaced from one another, a vane extending between the inner and outer walls and having leading and trailing edges, the vane including a plurality of discrete cavities between the leading and trailing edges and extending lengthwise of the vane for flowing cooling mediums, a pair of the cavities comprising cavities adjacent the leading edge and the trailing edge, respectively, for flowing cooling air to cool respective leading and trailing edges, at least two of the plurality of cavities disposed between the leading and trailing edge cavities and having insert sleeves therein, the sleeves extending substantially between the inner and outer walls and having openings therethrough, the inner wall including a plenum and the two cavities lying in communication with one another through the plenum, the outer wall having an inlet for flowing steam into one of the two cavities and an outlet for flowing spent cooling steam from another of the two cavities, the steam in the two cavities flowing through the openings in the insert sleeves for impingement-cooling side walls of the vane, the leading edge cavity including an air inlet, the leading edge having a plurality of holes for flowing cooling air from the leading edge cavity through the holes for film-cooling external surfaces of the leading edge of the vane.